/**
 * @file
 * Packet buffer management
 *
 * Packets are built from the pbuf data structure. It supports dynamic
 * memory allocation for packet contents or can reference externally
 * managed packet contents both in RAM and ROM. Quick allocation for
 * incoming packets is provided through pools with fixed sized pbufs.
 *
 * A packet may span over multiple pbufs, chained as a singly linked
 * list. This is called a "pbuf chain".
 *
 * Multiple packets may be queued, also using this singly linked list.
 * This is called a "packet queue". So, a packet queue consists of one
 * or more pbuf chains, each of which consist of one or more pbufs.
 * The differences between a pbuf chain and a packet queue are very
 * subtle. Currently, queues are only supported in a limited section
 * of lwIP, this is the etharp queueing code. Outside of this section
 * no packet queues are supported as of yet.
 *
 * The last pbuf of a packet has a ->tot_len field that equals the
 * ->len field. It can be found by traversing the list. If the last
 * pbuf of a packet has a ->next field other than NULL, more packets
 * are on the queue.
 *
 * Therefore, looping through a pbuf of a single packet, has an
 * loop end condition (tot_len == p->len), NOT (next == NULL).
 */

/*
 * Copyright (c) 2001-2003 Swedish Institute of Computer Science.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
 * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
 * OF SUCH DAMAGE.
 *
 * This file is part of the lwIP TCP/IP stack.
 *
 * Author: Adam Dunkels <adam@sics.se>
 *
 */

#include "lwip/opt.h"

#include "lwip/stats.h"

#include "lwip/def.h"
#include "lwip/mem.h"
#include "lwip/memp.h"
#include "lwip/pbuf.h"

#include "lwip/sys.h"

#include <sysclib.h>
#include <intrman.h>

#include "smsutils.h"

static u8_t pbuf_pool_memory[(PBUF_POOL_SIZE * MEM_ALIGN_SIZE(PBUF_POOL_BUFSIZE + sizeof(struct pbuf)))];

#if !SYS_LIGHTWEIGHT_PROT
static volatile u8_t pbuf_pool_free_lock, pbuf_pool_alloc_lock;
static sys_sem_t pbuf_pool_free_sem;
#endif

static struct pbuf *pbuf_pool = NULL;

/**
 * Initializes the pbuf module.
 *
 * A large part of memory is allocated for holding the pool of pbufs.
 * The size of the individual pbufs in the pool is given by the size
 * parameter, and the number of pbufs in the pool by the num parameter.
 *
 * After the memory has been allocated, the pbufs are set up. The
 * ->next pointer in each pbuf is set up to point to the next pbuf in
 * the pool.
 *
 */
void pbuf_init(void)
{
    struct pbuf *p, *q = NULL;
    u16_t i;

    pbuf_pool = (struct pbuf *)&pbuf_pool_memory[0];
    LWIP_ASSERT("pbuf_init: pool aligned", (long)pbuf_pool % MEM_ALIGNMENT == 0);

#if PBUF_STATS
    lwip_stats.pbuf.avail = PBUF_POOL_SIZE;
#endif /* PBUF_STATS */

    /* Set up ->next pointers to link the pbufs of the pool together */
    p = pbuf_pool;

    for (i = 0; i < PBUF_POOL_SIZE; ++i) {
        p->next = (struct pbuf *)((u8_t *)p + PBUF_POOL_BUFSIZE + sizeof(struct pbuf));
        p->len = p->tot_len = PBUF_POOL_BUFSIZE;
        p->payload = MEM_ALIGN((void *)((u8_t *)p + sizeof(struct pbuf)));
        p->flags = PBUF_FLAG_POOL;
        q = p;
        p = p->next;
    }

    /* The ->next pointer of last pbuf is NULL to indicate that there
     are no more pbufs in the pool */
    q->next = NULL;

#if !SYS_LIGHTWEIGHT_PROT
    pbuf_pool_alloc_lock = 0;
    pbuf_pool_free_lock = 0;
    pbuf_pool_free_sem = sys_sem_new(1);
#endif
}

/**
 * @internal only called from pbuf_alloc()
 */
static struct pbuf *
pbuf_pool_alloc(void)
{
    struct pbuf *p = NULL;

    SYS_ARCH_DECL_PROTECT(old_level);
    SYS_ARCH_PROTECT(old_level);

#if !SYS_LIGHTWEIGHT_PROT
    /* Next, check the actual pbuf pool, but if the pool is locked, we
     pretend to be out of buffers and return NULL. */
    if (pbuf_pool_free_lock) {
#if PBUF_STATS
        ++lwip_stats.pbuf.alloc_locked;
#endif /* PBUF_STATS */
        return NULL;
    }
    pbuf_pool_alloc_lock = 1;
    if (!pbuf_pool_free_lock) {
#endif /* SYS_LIGHTWEIGHT_PROT */
        p = pbuf_pool;
        if (p) {
            pbuf_pool = p->next;
        }
#if !SYS_LIGHTWEIGHT_PROT
#if PBUF_STATS
    } else {
        ++lwip_stats.pbuf.alloc_locked;
#endif /* PBUF_STATS */
    }
    pbuf_pool_alloc_lock = 0;
#endif /* SYS_LIGHTWEIGHT_PROT */

#if PBUF_STATS
    if (p != NULL) {
        ++lwip_stats.pbuf.used;
        if (lwip_stats.pbuf.used > lwip_stats.pbuf.max) {
            lwip_stats.pbuf.max = lwip_stats.pbuf.used;
        }
    }
#endif /* PBUF_STATS */

    SYS_ARCH_UNPROTECT(old_level);
    return p;
}


/**
 * Allocates a pbuf.
 *
 * The actual memory allocated for the pbuf is determined by the
 * layer at which the pbuf is allocated and the requested size
 * (from the size parameter).
 *
 * @param flag this parameter decides how and where the pbuf
 * should be allocated as follows:
 *
 * - PBUF_RAM: buffer memory for pbuf is allocated as one large
 *             chunk. This includes protocol headers as well.
 * - PBUF_ROM: no buffer memory is allocated for the pbuf, even for
 *             protocol headers. Additional headers must be prepended
 *             by allocating another pbuf and chain in to the front of
 *             the ROM pbuf. It is assumed that the memory used is really
 *             similar to ROM in that it is immutable and will not be
 *             changed. Memory which is dynamic should generally not
 *             be attached to PBUF_ROM pbufs. Use PBUF_REF instead.
 * - PBUF_REF: no buffer memory is allocated for the pbuf, even for
 *             protocol headers. It is assumed that the pbuf is only
 *             being used in a single thread. If the pbuf gets queued,
 *             then pbuf_take should be called to copy the buffer.
 * - PBUF_POOL: the pbuf is allocated as a pbuf chain, with pbufs from
 *              the pbuf pool that is allocated during pbuf_init().
 *
 * @return the allocated pbuf. If multiple pbufs where allocated, this
 * is the first pbuf of a pbuf chain.
 */

struct pbuf *
pbuf_alloc(pbuf_layer l, u16_t length, pbuf_flag flag)
{
    struct pbuf *p, *q, *r;
    u16_t offset;
    s32_t rem_len; /* remaining length */
    LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 3, ("pbuf_alloc(length=%u)\n", length));

    /* determine header offset */
    offset = 0;
    switch (l) {
        case PBUF_TRANSPORT:
            /* add room for transport (often TCP) layer header */
            offset += PBUF_TRANSPORT_HLEN;
        /* FALLTHROUGH */
        case PBUF_IP:
            /* add room for IP layer header */
            offset += PBUF_IP_HLEN;
        /* FALLTHROUGH */
        case PBUF_LINK:
            /* add room for link layer header */
            offset += PBUF_LINK_HLEN;
            break;
        case PBUF_RAW:
            break;
        default:
            LWIP_ASSERT("pbuf_alloc: bad pbuf layer", 0);
            return NULL;
    }

    switch (flag) {
        case PBUF_POOL:
            /* allocate head of pbuf chain into p */
            p = pbuf_pool_alloc();
            LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 3, ("pbuf_alloc: allocated pbuf %p\n", (void *)p));
            if (p == NULL) {
#if PBUF_STATS
                ++lwip_stats.pbuf.err;
#endif /* PBUF_STATS */
                return NULL;
            }
            p->next = NULL;

            /* make the payload pointer point 'offset' bytes into pbuf data memory */
            p->payload = MEM_ALIGN((void *)((u8_t *)p + (sizeof(struct pbuf) + offset)));
            LWIP_ASSERT("pbuf_alloc: pbuf p->payload properly aligned",
                        ((u32_t)p->payload % MEM_ALIGNMENT) == 0);
            /* the total length of the pbuf chain is the requested size */
            p->tot_len = length;
            /* set the length of the first pbuf in the chain */
            p->len = length > PBUF_POOL_BUFSIZE - offset ? PBUF_POOL_BUFSIZE - offset : length;
            /* set reference count (needed here in case we fail) */
            p->ref = 1;

            /* now allocate the tail of the pbuf chain */

            /* remember first pbuf for linkage in next iteration */
            r = p;
            /* remaining length to be allocated */
            rem_len = length - p->len;
            /* any remaining pbufs to be allocated? */
            while (rem_len > 0) {
                q = pbuf_pool_alloc();
                if (q == NULL) {
                    LWIP_DEBUGF(PBUF_DEBUG | 2, ("pbuf_alloc: Out of pbufs in pool.\n"));
#if PBUF_STATS
                    ++lwip_stats.pbuf.err;
#endif /* PBUF_STATS */
                    /* free chain so far allocated */
                    pbuf_free(p);
                    /* bail out unsuccesfully */
                    return NULL;
                }
                q->next = NULL;
                /* make previous pbuf point to this pbuf */
                r->next = q;
                /* set total length of this pbuf and next in chain */
                q->tot_len = rem_len;
                /* this pbuf length is pool size, unless smaller sized tail */
                q->len = rem_len > PBUF_POOL_BUFSIZE ? PBUF_POOL_BUFSIZE : rem_len;
                q->payload = (void *)((u8_t *)q + sizeof(struct pbuf));
                LWIP_ASSERT("pbuf_alloc: pbuf q->payload properly aligned",
                            ((u32_t)q->payload % MEM_ALIGNMENT) == 0);
                q->ref = 1;
                /* calculate remaining length to be allocated */
                rem_len -= q->len;
                /* remember this pbuf for linkage in next iteration */
                r = q;
            }
            /* end of chain */
            /*r->next = NULL;*/

            break;
        case PBUF_RAM:
            /* If pbuf is to be allocated in RAM, allocate memory for it. */
            p = mem_malloc(MEM_ALIGN_SIZE(sizeof(struct pbuf) + offset + length));
            if (p == NULL) {
                return NULL;
            }
            /* Set up internal structure of the pbuf. */

            // Boman666: The memory isn't allocated to allow payload to be aligned. If payload is aligned according to the commented out
            // line the last two bytes in payload will be located outside the allocated memoryblock. Which will have the effect of
            // screwing up the memory-allocation structures, causing a crash.
            //     p->payload = MEM_ALIGN((void *)((u8_t *)p + sizeof(struct pbuf) + offset));
            p->payload = (void *)((u8_t *)p + sizeof(struct pbuf) + offset);

            p->len = p->tot_len = length;
            p->next = NULL;
            p->flags = PBUF_FLAG_RAM;

            LWIP_ASSERT("pbuf_alloc: pbuf->payload properly aligned",
                        ((u32_t)p->payload % MEM_ALIGNMENT) == 0);
            break;
        /* pbuf references existing (static constant) ROM payload? */
        case PBUF_ROM:
        /* pbuf references existing (externally allocated) RAM payload? */
        case PBUF_REF:
            /* only allocate memory for the pbuf structure */
            p = memp_malloc(MEMP_PBUF);
            if (p == NULL) {
                LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_alloc: Could not allocate MEMP_PBUF for PBUF_%s.\n", flag == PBUF_ROM ? "ROM" : "REF"));
                return NULL;
            }
            /* caller must set this field properly, afterwards */
            p->payload = NULL;
            p->len = p->tot_len = length;
            p->next = NULL;
            p->flags = (flag == PBUF_ROM ? PBUF_FLAG_ROM : PBUF_FLAG_REF);
            break;
        default:
            LWIP_ASSERT("pbuf_alloc: erroneous flag", 0);
            return NULL;
    }
    /* set reference count */
    p->ref = 1;
    LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 3, ("pbuf_alloc(length=%u) == %p\n", length, (void *)p));
    return p;
}


#if PBUF_STATS
#define DEC_PBUF_STATS          \
    do {                        \
        --lwip_stats.pbuf.used; \
    } while (0)
#else /* PBUF_STATS */
#define DEC_PBUF_STATS
#endif /* PBUF_STATS */

#define PBUF_POOL_FAST_FREE(p) \
    do {                       \
        p->next = pbuf_pool;   \
        pbuf_pool = p;         \
        DEC_PBUF_STATS;        \
    } while (0)

#if SYS_LIGHTWEIGHT_PROT
#define PBUF_POOL_FREE(p)                 \
    do {                                  \
        SYS_ARCH_DECL_PROTECT(old_level); \
        SYS_ARCH_PROTECT(old_level);      \
        PBUF_POOL_FAST_FREE(p);           \
        SYS_ARCH_UNPROTECT(old_level);    \
    } while (0)
#else /* SYS_LIGHTWEIGHT_PROT */
#define PBUF_POOL_FREE(p)                   \
    do {                                    \
        sys_sem_wait(pbuf_pool_free_sem);   \
        PBUF_POOL_FAST_FREE(p);             \
        sys_sem_signal(pbuf_pool_free_sem); \
    } while (0)
#endif /* SYS_LIGHTWEIGHT_PROT */

/**
 * Shrink a pbuf chain to a desired length.
 *
 * @param p pbuf to shrink.
 * @param new_len desired new length of pbuf chain
 *
 * Depending on the desired length, the first few pbufs in a chain might
 * be skipped and left unchanged. The new last pbuf in the chain will be
 * resized, and any remaining pbufs will be freed.
 *
 * @note If the pbuf is ROM/REF, only the ->tot_len and ->len fields are adjusted.
 * @note May not be called on a packet queue.
 *
 * @bug Cannot grow the size of a pbuf (chain) (yet).
 */
void pbuf_realloc(struct pbuf *p, u16_t new_len)
{
    struct pbuf *q;
    u16_t rem_len; /* remaining length */
    s16_t grow;

    /* desired length larger than current length? */
    if (new_len >= p->tot_len) {
        /* enlarging not yet supported */
        return;
    }

    /* the pbuf chain grows by (new_len - p->tot_len) bytes
     * (which may be negative in case of shrinking) */
    grow = new_len - p->tot_len;

    /* first, step over any pbufs that should remain in the chain */
    rem_len = new_len;
    q = p;
    /* should this pbuf be kept? */
    while (rem_len > q->len) {
        /* decrease remaining length by pbuf length */
        rem_len -= q->len;
        /* decrease total length indicator */
        q->tot_len += grow;
        /* proceed to next pbuf in chain */
        q = q->next;
    }
    /* we have now reached the new last pbuf (in q) */
    /* rem_len == desired length for pbuf q */

    /* shrink allocated memory for PBUF_RAM */
    /* (other types merely adjust their length fields */
    if ((q->flags == PBUF_FLAG_RAM) && (rem_len != q->len)) {
        /* reallocate and adjust the length of the pbuf that will be split */
        mem_realloc(q, (u8_t *)q->payload - (u8_t *)q + rem_len);
    }
    /* adjust length fields for new last pbuf */
    q->len = rem_len;
    q->tot_len = q->len;

    /* any remaining pbufs in chain? */
    if (q->next != NULL) {
        /* free remaining pbufs in chain */
        pbuf_free(q->next);
    }
    /* q is last packet in chain */
    q->next = NULL;
}

/**
 * Adjusts the payload pointer to hide or reveal headers in the payload.
 *
 * Adjusts the ->payload pointer so that space for a header
 * (dis)appears in the pbuf payload.
 *
 * The ->payload, ->tot_len and ->len fields are adjusted.
 *
 * @param hdr_size Number of bytes to increment header size which
 * increases the size of the pbuf. New space is on the front.
 * (Using a negative value decreases the header size.)
 *
 * PBUF_ROM and PBUF_REF type buffers cannot have their sizes increased, so
 * the call will fail. A check is made that the increase in header size does
 * not move the payload pointer in front of the start of the buffer.
 * @return 1 on failure, 0 on success.
 *
 * @note May not be called on a packet queue.
 */
u8_t pbuf_header(struct pbuf *p, s16_t header_size)
{
    void *payload;

    /* remember current payload pointer */
    payload = p->payload;

    /* pbuf types containing payloads? */
    if (p->flags == PBUF_FLAG_RAM || p->flags == PBUF_FLAG_POOL) {
        /* set new payload pointer */
        p->payload = (u8_t *)p->payload - header_size;
        /* boundary check fails? */
        if ((u8_t *)p->payload < (u8_t *)p + sizeof(struct pbuf)) {
            LWIP_DEBUGF(PBUF_DEBUG | 2, ("pbuf_header: failed as %p < %p\n",
                                         (u8_t *)p->payload,
                                         (u8_t *)p + sizeof(struct pbuf))); /* restore old payload pointer */
            p->payload = payload;
            /* bail out unsuccesfully */
            return 1;
        }
        /* pbuf types refering to payloads? */
    } else if (p->flags == PBUF_FLAG_REF || p->flags == PBUF_FLAG_ROM) {
        /* hide a header in the payload? */
        if ((header_size < 0) && (header_size - p->len <= 0)) {
            /* increase payload pointer */
            p->payload = (u8_t *)p->payload - header_size;
        } else {
            /* cannot expand payload to front (yet!)
             * bail out unsuccesfully */
            return 1;
        }
    }
    LWIP_DEBUGF(PBUF_DEBUG, ("pbuf_header: old %p new %p (%d)\n", (void *)payload, (void *)p->payload, header_size));
    /* modify pbuf length fields */
    p->len += header_size;
    p->tot_len += header_size;

    return 0;
}

/**
 * Dereference a pbuf (chain) and deallocate any no-longer-used
 * pbufs at the head of this chain.
 *
 * Decrements the pbuf reference count. If it reaches
 * zero, the pbuf is deallocated.
 *
 * For a pbuf chain, this is repeated for each pbuf in the chain,
 * up to a pbuf which has a non-zero reference count after
 * decrementing. (This might de-allocate the whole chain.)
 *
 * @param pbuf The pbuf (chain) to be dereferenced.
 *
 * @return the number of pbufs that were de-allocated
 * from the head of the chain.
 *
 * @note MUST NOT be called on a packet queue.
 * @note the reference counter of a pbuf equals the number of pointers
 * that refer to the pbuf (or into the pbuf).
 *
 * @internal examples:
 *
 * Assuming existing chains a->b->c with the following reference
 * counts, calling pbuf_free(a) results in:
 *
 * 1->2->3 becomes ...1->3
 * 3->3->3 becomes 2->3->3
 * 1->1->2 becomes ......1
 * 2->1->1 becomes 1->1->1
 * 1->1->1 becomes .......
 *
 */
u8_t pbuf_free(struct pbuf *p)
{
    struct pbuf *q;
    u8_t count;
    SYS_ARCH_DECL_PROTECT(old_level);

    if (p == NULL) {
        LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_free(p == NULL) was called.\n"));
        return 0;
    }
    LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 3, ("pbuf_free(%p)\n", (void *)p));

    LWIP_ASSERT("pbuf_free: sane flags",
                p->flags == PBUF_FLAG_RAM || p->flags == PBUF_FLAG_ROM ||
                    p->flags == PBUF_FLAG_REF || p->flags == PBUF_FLAG_POOL);

    count = 0;
    /* Since decrementing ref cannot be guaranteed to be a single machine operation
     * we must protect it. Also, the later test of ref must be protected.
     */
    SYS_ARCH_PROTECT(old_level);
    /* de-allocate all consecutive pbufs from the head of the chain that
     * obtain a zero reference count after decrementing*/
    while (p != NULL) {
        /* all pbufs in a chain are referenced at least once */
        LWIP_ASSERT("pbuf_free: p->ref > 0", p->ref > 0);
        /* decrease reference count (number of pointers to pbuf) */
        p->ref--;
        /* this pbuf is no longer referenced to? */
        if (p->ref == 0) {
            /* remember next pbuf in chain for next iteration */
            q = p->next;
            LWIP_DEBUGF(PBUF_DEBUG | 2, ("pbuf_free: deallocating %p\n", (void *)p));
            /* is this a pbuf from the pool? */
            if (p->flags == PBUF_FLAG_POOL) {
                p->len = p->tot_len = PBUF_POOL_BUFSIZE;
                p->payload = (void *)((u8_t *)p + sizeof(struct pbuf));
                PBUF_POOL_FREE(p);
                /* a ROM or RAM referencing pbuf */
            } else if (p->flags == PBUF_FLAG_ROM || p->flags == PBUF_FLAG_REF) {
                memp_free(MEMP_PBUF, p);
                /* p->flags == PBUF_FLAG_RAM */
            } else {
                SYS_ARCH_UNPROTECT(old_level);
                mem_free(p);
                SYS_ARCH_PROTECT(old_level);
            }
            count++;
            /* proceed to next pbuf */
            p = q;
            /* p->ref > 0, this pbuf is still referenced to */
            /* (and so the remaining pbufs in chain as well) */
        } else {
            LWIP_DEBUGF(PBUF_DEBUG | 2, ("pbuf_free: %p has ref %u, ending here.\n", (void *)p, (unsigned int)p->ref));
            /* stop walking through chain */
            p = NULL;
        }
    }
    SYS_ARCH_UNPROTECT(old_level);
    /* return number of de-allocated pbufs */
    return count;
}

/**
 * Count number of pbufs in a chain
 *
 * @param p first pbuf of chain
 * @return the number of pbufs in a chain
 */

u8_t pbuf_clen(struct pbuf *p)
{
    u8_t len;

    len = 0;
    while (p != NULL) {
        ++len;
        p = p->next;
    }
    return len;
}

/**
 * Increment the reference count of the pbuf.
 *
 * @param p pbuf to increase reference counter of
 *
 */
void pbuf_ref(struct pbuf *p)
{
    SYS_ARCH_DECL_PROTECT(old_level);
    /* pbuf given? */
    if (p != NULL) {
        SYS_ARCH_PROTECT(old_level);
        ++(p->ref);
        SYS_ARCH_UNPROTECT(old_level);
    }
}

/**
 * Concatenate two pbufs (each may be a pbuf chain) and take over
 * the caller's reference of the tail pbuf.
 *
 * @note The caller MAY NOT reference the tail pbuf afterwards.
 * Use pbuf_chain() for that purpose.
 *
 * @see pbuf_chain()
 */

void pbuf_cat(struct pbuf *h, struct pbuf *t)
{
    struct pbuf *p;

    LWIP_ASSERT("h != NULL", h != NULL);
    LWIP_ASSERT("t != NULL", t != NULL);
    if ((h == NULL) || (t == NULL))
        return;

    /* proceed to last pbuf of chain */
    for (p = h; p->next != NULL; p = p->next) {
        /* add total length of second chain to all totals of first chain */
        p->tot_len += t->tot_len;
    }
    /* { p is last pbuf of first h chain, p->next == NULL } */
    LWIP_ASSERT("p->tot_len == p->len (of last pbuf in chain)", p->tot_len == p->len);
    /* add total length of second chain to last pbuf total of first chain */
    p->tot_len += t->tot_len;
    /* chain last pbuf of head (p) with first of tail (t) */
    p->next = t;
}

/**
 * Chain two pbufs (or pbuf chains) together.
 *
 * The caller MUST call pbuf_free(t) once it has stopped
 * using it. Use pbuf_cat() instead if you no longer use t.
 *
 * @param h head pbuf (chain)
 * @param t tail pbuf (chain)
 * @note The pbufs MUST belong to the same packet.
 * @note MAY NOT be called on a packet queue.
 *
 * The ->tot_len fields of all pbufs of the head chain are adjusted.
 * The ->next field of the last pbuf of the head chain is adjusted.
 * The ->ref field of the first pbuf of the tail chain is adjusted.
 *
 */
void pbuf_chain(struct pbuf *h, struct pbuf *t)
{
    pbuf_cat(h, t);
    /* t is now referenced by h */
    pbuf_ref(t);
    LWIP_DEBUGF(PBUF_DEBUG | DBG_FRESH | 2, ("pbuf_chain: %p references %p\n", (void *)h, (void *)t));
}

/**
 *
 * Create PBUF_POOL (or PBUF_RAM) copies of PBUF_REF pbufs.
 *
 * Used to queue packets on behalf of the lwIP stack, such as
 * ARP based queueing.
 *
 * Go through a pbuf chain and replace any PBUF_REF buffers
 * with PBUF_POOL (or PBUF_RAM) pbufs, each taking a copy of
 * the referenced data.
 *
 * @note You MUST explicitly use p = pbuf_take(p);
 * The pbuf you give as argument, may have been replaced
 * by pbuf_take()!
 *
 * @note Any replaced pbufs will be freed through pbuf_free().
 * This may deallocate them if they become no longer referenced.
 *
 * @param p Head of pbuf chain to process
 *
 * @return Pointer to head of pbuf chain
 */
struct pbuf *
pbuf_take(struct pbuf *p)
{
    struct pbuf *q, *prev, *head;
    LWIP_ASSERT("pbuf_take: p != NULL\n", p != NULL);
    LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 3, ("pbuf_take(%p)\n", (void *)p));

    prev = NULL;
    head = p;
    /* iterate through pbuf chain */
    do {
        /* pbuf is of type PBUF_REF? */
        if (p->flags == PBUF_FLAG_REF) {
            LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE, ("pbuf_take: encountered PBUF_REF %p\n", (void *)p));
            /* allocate a pbuf (w/ payload) fully in RAM */
            /* PBUF_POOL buffers are faster if we can use them */
            if (p->len <= PBUF_POOL_BUFSIZE) {
                q = pbuf_alloc(PBUF_RAW, p->len, PBUF_POOL);
                if (q == NULL)
                    LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_take: Could not allocate PBUF_POOL\n"));
            } else {
                /* no replacement pbuf yet */
                q = NULL;
                LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_take: PBUF_POOL too small to replace PBUF_REF\n"));
            }
            /* no (large enough) PBUF_POOL was available? retry with PBUF_RAM */
            if (q == NULL) {
                q = pbuf_alloc(PBUF_RAW, p->len, PBUF_RAM);
                if (q == NULL)
                    LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_take: Could not allocate PBUF_RAM\n"));
            }
            /* replacement pbuf could be allocated? */
            if (q != NULL) {
                /* copy p to q */
                /* copy successor */
                q->next = p->next;
                /* remove linkage from original pbuf */
                p->next = NULL;
                /* remove linkage to original pbuf */
                if (prev != NULL) {
                    /* prev->next == p at this point */
                    LWIP_ASSERT("prev->next == p", prev->next == p);
                    /* break chain and insert new pbuf instead */
                    prev->next = q;
                    /* prev == NULL, so we replaced the head pbuf of the chain */
                } else {
                    head = q;
                }
                /* copy pbuf payload */
                mips_memcpy(q->payload, p->payload, p->len);
                q->tot_len = p->tot_len;
                q->len = p->len;
                /* in case p was the first pbuf, it is no longer refered to by
                 * our caller, as the caller MUST do p = pbuf_take(p);
                 * in case p was not the first pbuf, it is no longer refered to
                 * by prev. we can safely free the pbuf here.
                 * (note that we have set p->next to NULL already so that
                 * we will not free the rest of the chain by accident.)
                 */
                pbuf_free(p);
                /* do not copy ref, since someone else might be using the old buffer */
                LWIP_DEBUGF(PBUF_DEBUG, ("pbuf_take: replaced PBUF_REF %p with %p\n", (void *)p, (void *)q));
                p = q;
            } else {
                /* deallocate chain */
                pbuf_free(head);
                LWIP_DEBUGF(PBUF_DEBUG | 2, ("pbuf_take: failed to allocate replacement pbuf for %p\n", (void *)p));
                return NULL;
            }
            /* p->flags != PBUF_FLAG_REF */
        } else {
            LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 1, ("pbuf_take: skipping pbuf not of type PBUF_REF\n"));
        }
        /* remember this pbuf */
        prev = p;
        /* proceed to next pbuf in original chain */
        p = p->next;
    } while (p);
    LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 1, ("pbuf_take: end of chain reached.\n"));

    return head;
}
